Method to construct an electric machine having a stator winding with rigid bars

ABSTRACT

A method to construct an electric machine provided with: a stator having a magnetic core longitudinally crossed by a plurality of stator slots; a stator winding having a series of rigid bars, which are “U”-shaped and are inserted through the stator slots defining an entry side, in which the cusps of the “U”-shaped bars are placed, and an exit side, in which the legs of the “U”-shaped bars are placed; the construction method comprises the steps of electrically connecting the ends of at least two non-adjacent legs to one another by means of a connection bridge, which is made of a flat plate arranged perpendicular to the central axis of rotation and has, for each leg, a corresponding “U”-shaped seat, which is suited to receive the end of the leg.

TECHNICAL FIELD

The present invention relates to a method to construct an electricmachine having a stator winding with rigid bars.

PRIOR ART

Patent application EP2437378A1 describes an electric machine having astator winding with rigid bars. In a stator winding made of rigid bars,a series of rigid bars are used, which are initially shaped into a “U”and are then axially inserted into the stator slots, thus defining anentry side, in which the cusps of the “U”-shaped bars are placed, and anexit side, in which the legs (i.e. the straight portions) of the“U”-shaped bars are placed. Once the bars have been inserted into thestator slots, the legs on the exit side are bent and then the free endsof the legs are connected to one another by means of welding so as toform the electrical paths of the stator winding.

On the exit side, in order to connect the ends of some legs to oneanother, connection bridges are used, each of which is made of a flatplate arranged perpendicular to the central axis of rotation and has,for each leg, a corresponding “U”-shaped seat, which is suited toreceive the end of the leg itself.

However, it was found out that coupling the connection bridges to thestator winding is relatively complex (and therefore difficult to beautomated) because it requires some adjustment (performed manually by anoperator) in order to properly insert the ends of the legs into thecorresponding seats of the connection bridge.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a method to constructan electric machine having a stator winding with rigid bars, said methodbeing conceived to be simple and cheap to be produced and, at the sametime, to eliminate the drawbacks described above.

According to the present invention, a construction method is provided toconstruct an electric machine having a stator winding with rigid barsaccording to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to theaccompanying drawings, which show a non-limiting embodiment thereof,wherein:

FIG. 1 is a schematic, cross-sectional view, with parts removed forgreater clarity, of an electric machine according to the presentinvention;

FIG. 2 is a schematic, perspective view, with parts removed for greaterclarity, of a stator of the electric machine of FIG. 1;

FIG. 3 is a schematic, perspective view of a “U”-shaped rigid bar thatis part of a stator winding of the stator of FIG. 2;

FIGS. 4 and 5 are two different perspective views on a larger scale of acusp of the rigid bar of FIG. 3;

FIG. 6 is a perspective view highlighting the planes of curvature of acusp of the rigid bar of FIG. 3;

FIG. 7 is a perspective view of an entry side of the stator of FIG. 2;

FIG. 8 is a perspective view of the stator of FIG. 2 during theconstruction of the stator winding;

FIGS. 9 and 10 are two views on a larger scale of respective details ofFIG. 8;

FIG. 11 is a schematic view showing how corresponding legs of twodifferent “U”-shaped bars of the stator winding are twisted into a“Z”-shape;

FIG. 12 is a further perspective view of part of the stator of FIG. 2during the construction of the stator winding;

FIG. 13 is a perspective view of a support element which is coupled toconnection bridges of the stator winding;

FIG. 14 is an exploded perspective view of the support element of FIG.13;

FIGS. 15-18 are further perspective views of part of the stator of FIG.2 during the construction of the stator winding;

FIG. 19 is a schematic view of a welding station which is used duringthe construction of the stator winding; and

FIGS. 20-25 are six schematic views showing the steps involved in thewelding of two adjacent legs of the stator winding.

PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, number 1 indicates, as a whole, a synchronous electricmachine for the automotive industry of the reversible type (i.e. whichcan act both as an electric motor, by absorbing electrical energy andgenerating a mechanical driving torque, and as an electric generator, byabsorbing mechanical energy and generating electrical energy). Theelectric machine 1 comprises a shaft 2, which is mounted in a rotatorymanner so as to rotate around a central axis 3 of rotation, a permanentmagnet rotor 4 splined to the shaft 2 so as to rotate together with theshaft 2 itself, and a cylindrical tubular stator 5 arranged around therotor 4 so as to surround the rotor 4 itself.

The stator 5 comprises a magnetic core 6, which consists of a series oflaminations bound into packs and has a tubular shape with a centralhole; the magnetic core 6 is longitudinally crossed by thirty-six statorslots 7 which are uniformly distributed along the internal side of themagnetic core 6 and house a three-phase stator winding 8 (shown in FIG.2); obviously, the number of slots 7 may be different (for example,forty-eight or seventy-two stator slots 7). In the embodiment shown inthe accompanying drawings, the three-phase stator winding 8 isdistributed in thirty-six stator slots 7 and the permanent magnet rotor4 comprises six poles; as a consequence, the synchronous electricmachine 1 has two slots for each pole and for each phase.

According to FIG. 2, the three-phase stator winding 8 comprises a seriesof “U”-shaped rigid bars 9, each comprising two legs 10 which areconnected to one another by a cusp 11 (as shown more in detail in FIGS.3, 4 and 5); the two legs 10 of the same bar 9 constitute twocorresponding conductors of the stator winding 8.

As shown more in detail in FIG. 6, the cusp 11 of each “U”-shaped bar 9has a “three-dimensional” fold, i.e. on two planes A and B that areperpendicular to one another. In particular, the cusp 11 of each“U”-shaped bar 9 has a fold that follows plane A, which is substantiallyparallel to the lying plane defined by the “U”-shaped bar 9 and is“S”-shaped; furthermore, the cusp 11 of each “U”-shaped bar 9 has afurther fold that follows plane B, which is substantially perpendicularto the lying plane defined by the “U”-shaped bar 9 (i.e. it issubstantially perpendicular to plane A) and is “U”-shaped.

As clearly shown in FIG. 6, plane A laterally consists of two portionsof cylindrical surface, which are centred on the central axis 3 ofrotation of the electric machine 1 and have slightly different radii(since a leg 10 of the “U”-shaped bar 9 is inserted into a stator slot 7placed in a more internal position, namely in correspondence to asmaller radius, while the other leg 10 of the “U”-shaped bar 9 isinserted into a stator slot 7 placed in a more external position, namelyin correspondence to a bigger radius); furthermore, in plane A the twoportions of cylindrical surface arranged on the sides are centrallyjoined to one another by an “S”-shaped central portion. As clearly shownin FIG. 6, plane B has a “U”-shape, which causes the corresponding bar 9to take on a “U”-shape. The “S”-shaped fold along plane A causes thecorresponding bar 9 to take on the shape needed both to allow the twolegs 10 to be inserted into the corresponding stator slots 7 indifferent positions (a more internal and a more external position) andto allow the cusps 11 of all “U”-shaped bars 9 to be arranged one besidethe other without any mechanical interference with one another (as shownin FIG. 7). It should be pointed out that, in each “U”-shaped bar 9, thetwo legs 10 are inclined relative to one another (i.e. the two legs 10form an acute angle between them) and each leg 10 is oriented so as to“face” the central axis 3 of rotation of the electric machine 1.

Thanks to the configuration of the “U”-shaped bars 9 described above, itis possible to build the whole stator winding 8 by bending the bars 9 incorrespondence to the cusps 11 in a single bending step before insertingthe bars 9 into the corresponding stator slots 7; in other words, oncethe “U”-shaped bars 9 have been inserted into the corresponding statorslots 7, the cusps 11 are not subject to any further bending.Furthermore, thanks to the configuration of the “U”-shaped bars 9described above, it is possible to insert the “U”-shaped bars 9 into thecorresponding stator slots 7 extremely easily (i.e. with little effort),since the configuration prevents all mechanical interferences betweenthe “U”-shaped bars 9, which would require the insertion of the bars tobe forced (namely the bars would have to be pushed with force) in orderto slightly deform the “U”-shaped bars 9 themselves; in this way, theconstruction of the stator winding 8 is quick and easy and, above all,there is no risk of damage to the external insulation of the “U”-shapedbars 9.

Furthermore, thanks to the configuration of the “U”-shaped bars 9described above, it is possible to reduce the length (and therefore thetotal weight) of the cusps 11, in other words it is possible to reducethe quantity of copper to be found on the outside of the stator slots 7(the copper to be found on the outside of the stator slots 7 is neededto enable electrical current to circulate in the stator winding 8, butit is completely useless for the purpose of delivering the mechanicaltorque to the shaft 2). In other words, the configuration of the“U”-shaped bars 9 described above allows the length (and therefore thetotal weight) of the cusps 11 to be optimized (minimized).

According to FIG. 2, the “U”-shaped bars 9 are inserted through thestator slots 7 defining an entry side 12, in which the cusps 11 of the“U”-shaped bars 9 are placed, and an exit side 13, in which the legs 10of the “U”-shaped bars 9 are placed. In particular, each slotaccommodates four legs 10 (namely four conductors of the stator winding8) belonging to four corresponding “U”-shaped rigid bars 9 which aredifferent from one another. The ends of the legs 10 of the “U”-shapedbars 9 are electrically connected to one another (welded) so as to formthe electrical paths of the stator winding 8.

According to FIG. 2, the three-phase stator winding 8 comprises threepower terminals 14 which constitute the electrical interface with theoutside of the stator winding 8 and are electrically connected to anelectronic power converter (not shown) which controls the synchronouselectric machine 1.

According to a preferred (though not limiting) embodiment shown in FIG.2, an insulating layer S with an annular shape and made of anelectrically insulating material (typically “Nomex®” insulating paper)is interposed between the two internal circles and the two externalcircles of legs 10 coming out from the exit side 13 of the statormagnetic core 6. Alternatively, it is possible to interpose threeinsulating layers S with an annular shape and made of an electricallyinsulating material between the four circles of legs 10, or theinsulating layers S with an annular shape and made of an electricallyinsulating material may not be present at all.

With reference to FIGS. 8-18, below you can find a description of theway in which the stator winding 8 is built.

At first (and as described above) the bars 9 are bent in the middle by180° so as to become “U”-shaped and take on the shape shown in FIGS.3-6. This bending operation is preferably carried out by means of anelectronically controlled machine tool which gives each bar 9 its finalshape in one single operation. The cusps 11 of the “U”-shaped bars 9 areconfigured in such a way that they are spaced apart by the same numberof slots (that is six stator slots 7 in the embodiment shown in theaccompanying drawings); in other words, between two legs 10 of the same“U”-shaped bar 9 there is always a skip of six stator slots 7.Furthermore, the legs 10 of the “U”-shaped bars 9 can all have the samelength, or they can have different lengths (i.e. the legs 10 of some“U”-shaped bars 9 are slightly longer or shorter than the legs 10 ofother “U”-shaped bars 9); the solution according to which the legs 10 ofthe “U”-shaped bars 9 all have the same length involves a higherconsumption of copper, although it is easier to handle, and is thereforenormally preferred in case of smaller production volumes.

Once the “U”-shaped bars 9 are ready (in particular, in the statorwinding 8 shown in the accompanying figures there are seventy-two“U”-shaped bars 9), each “U”-shaped bar 9 is individually inserted (i.e.alone) into the corresponding two stator slots 7 (namely the two legs 10of the “U”-shaped bar 9 are axially inserted into the corresponding twostator slots 7). In other words, only one “U”-shaped bar 9 at a time isinserted into two non-adjacent stator slots 7. Preferably, beforeinserting the “U”-shaped bars 9 into the stator slots 7, each statorslot 7 is internally “coated” with an insulating layer (not shown) madeof an electrically insulating material (typically “Nomex®” insulatingpaper).

Once all the seventy-two “U”-shaped bars 9 have been inserted into thestator slots 7, the corresponding legs 10 coming out from the exit side13 of the magnetic core 6 are twisted through relative double folds soas to take on a “Z”-shape as shown in FIG. 8. Alternatively, at firstonly a part (normally half) of the “U”-shaped bars 9 is inserted and,once this first part of the “U”-shaped bars 9 has been inserted, thecorresponding legs 10 coming out from the exit side 13 of the magneticcore 6 are twisted through relative double folds so as to take on a“Z”-shape; subsequently, the remaining part (normally the remaininghalf) of the “U”-shaped bars 9 is also inserted and, once the remainingpart of the “U”-shaped bars 9 has been inserted, the corresponding legs10 coming out from the exit side 13 of the magnetic core 6 are twistedthrough relative double folds so as to take on a “Z”-shape.

According to a preferred embodiment, during the twisting of the legs 10of the “U”-shaped bars 9, the legs 10 are not twisted all at once, theyare twisted in groups; in other words, the legs 10 of the “U”-shapedbars 9 are divided into at least two groups which are twisted in twoconsecutive operations (i.e. not simultaneously). While a first group oflegs 10 is being twisted, the legs 10 of the second group, which will betwisted at a later time, are temporarily axially moved along the statorslots 7, so as to reduce the length of the portion coming out from thestator slots 7 on the exit side 13 (typically until the length ofportion of the legs 10 coming out from the stator slots 7 on the exitside 13 is zero or almost zero). In this way the legs 10 belonging tothe second group are not an obstacle to the legs 10 of the first groupbeing twisted, and at the same time the legs 10 of the second groupengage the stator slots 7 so as to help keep the legs 10 of the firstgroup still during twisting. Once the first group of legs 10 have beentwisted, the legs 10 of the second group are axially moved in theopposite direction so as to go back to their original position and betwisted as well. In this way, the twisting of the legs 10 of the firstgroup is made easier, because said twisting takes place without theobstacle posed by the legs 10 of the second group. The twisting of thelegs 10 of the second group is easier anyway, since it takes place oncethe legs 10 of the first group have already been twisted and thereforetake up much less axial space.

Once the twisting of the legs 10 coming out from the exit side 13 of themagnetic core 6 has been completed, and only if necessary, some legs 10are cut (i.e. trimmed) so as to give them the desired length.

As shown more in detail in the enlargements of FIGS. 9 and 10, eachdouble fold of a leg 10 comprises an internal fold 15 (namely closer tothe magnetic core 6 of the stator 5) in one direction and an externalfold 16 (namely farther away from the magnetic core 6 of the stator 5)in the opposite direction. By way of example, for each leg 10, theinternal fold 15 has a width of about 60° in a direction and theexternal fold 15 has a width of about 60° in the opposite direction.Each stator slot 7 accommodates four corresponding legs 10 (belonging tofour different corresponding “U”-shaped bars) which are twisted inalternately opposite directions through relative “Z”-shaped doublefolds. In other words, half of the legs 10 are twisted in a clockwisedirection at a given angle, while the other half of the legs 10 aretwisted in a counter-clockwise direction (that is in the oppositedirection) at the same angle.

According to FIG. 11, once the “Z”-shaped double twisting has beenperformed, each leg 10 has an initial segment 10 a which is axiallyoriented (i.e. parallel to the central axis 3 of rotation of theelectric machine 1, and therefore parallel to the stator slots 7), aninclined intermediate segment 10 b and a final segment 10 c which isaxially oriented as well; the intermediate segment 10 b joins theinitial segment 10 a and the final segment 10 c, which are parallel toeach other, together. In each leg 10, the intermediate segment 10 b isinclined by the same angle α (in the embodiment shown, about 60°) bothrelative to the initial segment 10 a and relative to the final segment10 c; in other words, starting from the straight leg 10, the leg 10 isinitially twisted at the bottom by an angle α so as to form the internalfold 15 (between the initial segment 10 a and the intermediate segment10 b) and the leg 10 is then twisted at the top by the angle α so as toform the external fold 16 (between the intermediate segment 10 b and thefinal segment 10 c).

In each leg 10, due to the “Z”-shaped double fold, the initial segment10 a and the final segment 10 c are parallel to one another (i.e. theyare both axially oriented) and spaced apart from one another by a pitchthat can take on a higher value P1 or a lower value P2 (in theembodiment shown in the accompanying drawings, pitch P1 corresponds tothe distance existing between three stator slots 7, while pitch P2corresponds to the distance existing between two and a half stator slots7). In order to differentiate pitch P1 from pitch P2, it is not thetwisting angle α that is varied (in fact, all legs 10 are always twistedby the same angle α in order to form the double folds), but rather theextension of the intermediate segment 10 b (and, accordingly, the finalsegment 10 c has a different length, too); in other words, in order toobtain the longer pitch P1, a longer intermediate segment 10 b is used,while in order to obtain the shorter pitch P2, a shorter intermediatesegment 10 b is used, while always keeping the twisting angle α used toform the double folds unchanged.

As already mentioned above, all “U”-shaped bars 9 are spaced apart bythe same number of slots, i.e. between the two legs 10 of all “U”-shapedbars 9 there is a skip of six stator slots 7; as a consequence, in orderto form the electrical paths of the stator winding 8, it is necessarythat the legs 10 of the “U”-shaped bars 9 are differentiated in their“Z”-shaped double folds: as described above with reference to FIG. 11,the majority of the legs 10 of the “U”-shaped bars 9 have a standard“Z”-shaped double fold with pitch P1 (corresponding to the distanceexisting between three stator slots 7), while a small part of the legs10 of the “U”-shaped bars 9 have a shortened “Z”-shaped double fold withpitch P2 (corresponding to the distance existing between two and a halfstator slots 7). As an alternative to the shortened “Z”-shaped doublefold with pitch P2 (corresponding to the distance existing between twoand a half stator slots 7), it may be possible to use an extendedversion of the “Z”-shaped double fold with a pitch P1 (corresponding tothe distance existing between three stator slots 7).

As shown in FIG. 8, the legs 10 of the “U”-shaped bars 9 having theshortened “Z”-shaped double fold (pitch P2) are concentrated in aninterconnection area 17 of the stator winding 8, which has an extensionof about one fourth of the whole stator winding 8 and which alsoaccommodates the power terminals 14.

Once the legs 10 of the “U”-shaped bars 9 have been twisted, the upperends of the legs 10 of the “U”-shaped bars 9 are electrically connectedto one another (by means of laser welding, as described more in detailbelow) so as to form the electrical paths of the stator winding 8.

In correspondence to the interconnection area 17 of the stator winding8, some legs 10 have longer (more extended) terminal ends 10 c, so as toform some of the electrical connections of the stator winding 8 asdescribed below.

As shown in FIG. 12, in order to allow the electrical paths of thestator winding 8 to be formed in the part of the stator winding 8corresponding to each phase, the ends of at least one pair ofnon-adjacent legs 10 (which therefore can not be directly welded to oneanother) are electrically connected to one another by means of aconnection bridge 18. As shown in FIG. 12, in the stator winding 8pictured in the accompanying drawings each phase comprises twocorresponding connection bridges 18, therefore the whole stator winding8 has a total of six connection bridges 18. According to a preferredembodiment, each connection bridge 18 is made of a flat plate arrangedperpendicular to the central axis 3 of rotation of the electric machine1 and has two opposite seats 19 that engage the ends of the respectivelegs 10. Once the ends of the legs 10 have been coupled to thecorresponding connection bridges 18, the end of each leg 10 is welded tothe corresponding connection bridge 18 by means of the laser technique(as described more in detail below).

According to a preferred embodiment shown in FIGS. 12, 13 and 14, eachconnection bridge 18 comprises seats 19, each of which is “U”-shaped andsuited to receive and accommodate the end of a corresponding leg 10.Each seat 19 initially has a larger cross section (beyond the toleranceneeded to simply insert the end of the corresponding leg 10) than thecross section of the corresponding leg 10, so that the leg 10 itself canbe inserted into the seat 19 with a certain clearance (beyond thetolerance needed to simply insert the end of the corresponding leg 10)and be “loose” inside the seat 19 itself; in other words, each seat 19is initially (considerably) larger than the corresponding leg 10 inorder to accommodate the leg 10 with a significant clearance andtherefore allow the end of the leg 10 to be inserted extremely easily.Once the end of a leg 10 has been inserted into a corresponding seat 19of a connection bridge 18, the seat 19 is caused to shrink by means ofplastic deformation, so as for it to be tight around the leg 10 (i.e. tofirmly adhere to the leg 10); in particular, the seat 19 is deformedtowards the leg 10 by means of a suitable gripper, so as to be in closecontact with (also exerting a certain pressure on) the leg 10 itself.

Each seat 19 is “U”-shaped and is therefore delimited on three sides(the fourth side is open) by two lateral walls and by a rear wall whichis interposed between the two lateral walls (i.e. the two lateral wallsare arranged on opposite sides of the rear wall). At first (i.e. beforethe corresponding leg 10 is inserted into the seat 19) the two lateralwalls are not parallel to one another (i.e. they tend to diverge fromeach other) and each forms a corresponding obtuse angle with the rearwall; once the end of the leg 10 has been inserted into the seat 19, theseat 19 is caused to shrink, thus bringing the two lateral walls closerto each other and therefore the two lateral walls become parallel toeach other and each forms a corresponding right angle with the rearwall.

In this way, on the one hand it is possible to easily insert the ends ofthe legs 10 into the corresponding seats 19, and on the other hand it ispossible to ensure a firm mechanical coupling without any gaps betweenthe ends of the legs 10 and the corresponding seats 19, so as to ensurethat, later on, the ends of the legs 10 and the corresponding seats 19are properly welded to each other. In other words, thanks to the factthat the seats 19 of each connection bridge 18 are initially larger thanthe corresponding legs 10, coupling the connection bridge 18 to thestator winding 8 proves to be simple and quick (it can therefore beeasily automated); furthermore, thanks to the fact that the seats 19 ofeach connection bridge 18 are plastically deformed so as for them to betight around the corresponding legs 10, it is possible to obtain anoptimal (mechanical and therefore electric) contact between theconnection bridge 18 and the corresponding legs 10.

In order to allow the seats 19 to be easily deformed around the ends ofthe corresponding legs 10, the connection bridge 18 is made up of acentral body with a substantially constant cross-section and of a seriesof appendages, which project from the central body and are perpendicularto the central body itself; each seat 19 is delimited, on the rear side,by the central body and is laterally delimited by a pair of appendagesthat are arranged one in front of the other. Preferably, in each seat19, one appendage is linear (i.e. it has a linear shape in its planview) and the other appendage is “L”-shaped (i.e. it is “L”-shaped inits plan view); in this way, the “L”-shaped appendage can embrace theend of the corresponding leg 10 on two sides. At first (i.e. before theend of the corresponding leg 10 is inserted), an appendage (in theembodiments shown in the accompanying drawings, the linear appendage)forms a right angle (namely an angle of 90°) with the central body,while the other appendage (in the embodiments shown in the accompanyingdrawings, the “L”-shaped appendage) initially forms an obtuse angle(namely an angle larger than 90°) with the central body; the deformationof each seat 19 essentially (but not exclusively) involves the appendagethat initially forms an obtuse angle with the central body. In otherwords, the appendage initially forming an obtuse angle with the centralbody has wider deformation margins, because, before the deformationtakes place, it is slightly farther away from the end of thecorresponding leg 10 than the other appendage initially forming a rightangle with the central body.

The electrical connection between two adjacent ends of two legs 10 isnormally established between two legs 10 which are spaced apart from oneanother by six stator slots 7 (i.e. with a skip of six stator slots 7obtained by means of the standard “Z”-shaped double fold with pitch P1,which is equal to three slots); furthermore, every time the magneticcore 6 of the stator makes a complete revolution, there must be a skipof five stator slots 7 (obtained by means of the shortened “Z”-shapeddouble fold with pitch P2, which is equal to two and a half slots) or askip of seven stator slots 7 (obtained by means of the connectionbridges 18), so as to follow the electrical paths of the stator winding8.

According to a preferred embodiment shown in FIGS. 12-15, an insulatingsupport 20 is provided, which houses the connection bridges 18 and isarranged under the connection bridges 18. According to a preferredembodiment, the insulating support 20 is made of an electricallyinsulating plastic material. The insulating support 20 comprises a base21, which is shaped as a circular arc and has a lower surface, whichrests against the ends of the legs 10 arranged under it, and an uppersurface, on which the connection bridges 18 rest. The base 21 of theinsulating support 20 has a plurality of through holes 22, into whichsome legs are inserted 10, which have to pass through the insulatingsupport 20 in order to be connected to the connection bridges 18 or tobe connected to further elements (described below) arranged above theconnection bridges 18.

The insulating support 20 comprises a plurality of positioning elements23, which project downwards from the lower surface of the base 21 andare interlocked in gaps available between the ends of the legs 10arranged under them (as shown in FIG. 12), so as to both ensure acorrect positioning of the insulating support 20, and mechanicallyconstrain the insulating support 20 to the underlying part of the statorwinding 8. In the preferred embodiment shown in the accompanyingdrawings, the positioning elements 23 are arranged perpendicular to thebase 21. The insulating support 20 comprises a plurality of columns 24,which project upwards from the upper surface of the base 21 (i.e. fromthe opposite side and in the opposite direction with respect to thepositioning elements 23) and provide a mechanical support for furtherelements (described below) arranged above the insulating support 20. Inthe preferred embodiment shown in the accompanying drawings, the columns24 are arranged circumferentially both on the internal edge and on theexternal edge of the insulating support 20.

Inside the insulating support 20 there are seats 25 (shown in FIG. 14),each of which is suited to receive without any substantial clearance andto mechanically lock a corresponding connection bridge 18 in the correctposition. Each seat 25 is made up of restraining elements 26, whichproject upwards from the upper surface of the base 21 and are arrangedin correspondence to the edge of the corresponding connection bridge 18;at least part of the restraining elements 26 have a tooth 27, whichrests on an upper surface of the corresponding connection bridge 18following an elastic deformation of the restraining element 26 itself.In other words, each connection bridge 18 is forcedly inserted into thecorresponding seat 25, so as to determine an elastic deformation of atleast part of the restraining elements 26; accordingly, in order toextract a connection bridge 18 from the corresponding seat 25, it isnecessary to elastically deform at least part of the restrainingelements 26.

The use of the insulating support 20 makes the coupling of theconnection bridges 18 to the stator winding 8 considerably easier; inparticular, it is possible to pre-assemble (and lock in an interlockingmanner) the connection bridges 18 into the corresponding seats 25 of theinsulating support 20 far from the stator winding 8 and thereforecouple, with a single operation, the insulating support 20, togetherwith all the connection bridges 18, to the stator winding 8.Furthermore, the insulating support 20 allows the connection bridges 18to be firmly kept in the desired position during the construction of thestator winding 8 (i.e. before the seats 19 of the connection bridges 18are tightened around the ends of the corresponding legs 10 and beforethe connection bridges 18 are welded to the ends of the correspondinglegs 10).

According to FIG. 15, the three ends of the respective legs 10 whichform a star-centre of the stator winding 8 are electrically connected toone another by means of a connection bridge 28, which is made of a flatplate arranged perpendicular to the central axis 3 of rotation of theelectric machine 1 and has three seats, which engage the ends of therespective legs 10. The connection bridges 28 are similar to theconnection bridges 20 described above and therefore comprise respectiveseats, which are plastically deformed once the ends of the correspondinglegs 10 have been inserted into them. As shown in FIG. 15, the statorwinding 8 shown in the accompanying drawings comprises twostar-connections which, in turn, are connected in parallel to oneanother (as described more in detail below). The two connection bridges28 are arranged above the six connection bridges 18. According to apreferred embodiment shown in the accompanying drawings, an insulatingsupport 29 is provided, which houses the connection bridges 28, isarranged under the connection bridges 28, and is similar to theinsulating support 20.

In particular, the insulating support 29 rests, on the lower side, onthe top of the columns 24 of the insulating support 20. The onlysubstantial difference (obviously apart from the shape and thearrangement of the seats 25) between the insulating support 29 and theinsulating support 20 is the fact that the insulating support 29 doesnot have the positioning elements 23, since it rests on the columns 24of the underlying insulating support 20. Other than that, the insulatingsupport 29 comprises a base which is crossed by through holes like theinsulating support 20, it comprises columns like the insulating support20, and it has seats provided with restraining elements, which, in case,may have teeth, like the insulating support 20.

According to FIG. 16, a connection bridge 30 is provided, which has apower terminal 14, is arranged above the connection bridges 28, and ismade of a flat plate arranged perpendicular to the central axis 3 ofrotation of the electric machine 1 and has two seats, which engage theends of the respective legs 10. The connection bridge 30 is similar tothe connection bridges 20 described above and therefore comprisesrespective seats, which are plastically deformed once the ends of thecorresponding legs 10 have been inserted into them. According to apreferred embodiment shown in the accompanying drawings, an insulatingsupport 31 is provided, which houses the connection bridge 30, isarranged under the connection bridge 30, and is similar to theinsulating supports 20 and 29. In particular, the insulating support 31rests, on the lower side, on the top of the columns of the insulatingsupport 29. The only substantial difference (obviously apart from theshape and the arrangement of the seats 25) between the insulatingsupport 31 and the insulating support 20 is the fact that the insulatingsupport 31 does not have the positioning elements 23, since it rests onthe columns of the underlying insulating support 29. Other than that,the insulating support 31 comprises a base which is crossed by throughholes like the insulating support 20, it comprises columns like theinsulating support 20, and it has seats provided with restrainingelements, which, in case, may have teeth, like the insulating support20.

According to FIG. 17, a connection bridge 32 is provided, which has apower terminal 14, is arranged above the connection bridge 30, and ismade of a flat plate arranged perpendicular to the central axis 3 ofrotation of the electric machine 1 and has two seats, which engage theends of the respective legs 10. The connection bridge 32 is similar tothe connection bridges 20 described above and therefore comprisesrespective seats, which are plastically deformed once the ends of thecorresponding legs 10 have been inserted into them. According to apreferred embodiment shown in the accompanying drawings, an insulatingsupport 33 is provided, which houses the connection bridge 32, isarranged under the connection bridge 32, and is similar to theinsulating supports 20, 29 and 31. In particular, the insulating support33 rests, on the lower side, on the top of the columns of the insulatingsupport 31. The only substantial difference (obviously apart from theshape and the arrangement of the seats 25) between the insulatingsupport 33 and the insulating support 20 is the fact that the insulatingsupport 33 does not have the positioning elements 23, since it rests onthe columns of the underlying insulating support 31. Other than that,the insulating support 33 comprises a base which is crossed by throughholes like the insulating support 20, it comprises columns like theinsulating support 20, and it has seats provided with restrainingelements, which, in case, may have teeth, like the insulating support20.

According to FIG. 18, a connection bridge 34 is provided, which has apower terminal 14, is arranged above the connection bridge 32, and ismade of a flat plate arranged perpendicular to the central axis 3 ofrotation of the electric machine 1 and has two seats, which engage theends of the respective legs 10. The connection bridge 34 is similar tothe connection bridges 20 described above and therefore comprisesrespective seats, which are plastically deformed once the ends of thecorresponding legs 10 have been inserted into them. According to apreferred embodiment shown in the accompanying drawings, an insulatingsupport 35 is provided, which houses the connection bridge 34, isarranged under the connection bridge 34, and is similar to theinsulating supports 20, 29, 31 and 33. In particular, the insulatingsupport 35 rests, on the lower side, on the top of the columns of theinsulating support 33. The only substantial difference (obviously apartfrom the shape and the arrangement of the seats 25) between theinsulating support 35 and the insulating support 20 is the fact that theinsulating support 35 does not have the positioning elements 23, sinceit rests on the columns of the underlying insulating support 33. Otherthan that, the insulating support 35 comprises a base which is crossedby through holes like the insulating support 20, it comprises columnslike the insulating support 20, and it has seats provided withrestraining elements, which, in case, may have teeth, like theinsulating support 20.

Thanks to the connection bridges 30, 32 and 34, each power terminal 14is electrically connected to the ends of two legs 10 which constituterespective terminals of a star connection of the stator winding 8 andwhich project beyond the corresponding support elements 31, 33 and 35;in this way, the connection bridges 30, 32 and 34 establish theconnection in parallel between the two star-connections of the statorwinding 8.

In the embodiment shown in the accompanying drawings, five connectionbridges 18, 28, 30, 32 and 34 are used, which are arranged one on top ofthe other (i.e. piled on top of one another); according to otherperfectly equivalent embodiments, it is possible to use a differentnumber (for example, four or six) of connection bridges.

With reference to FIGS. 19-25, below you can find a description of theways in which the welds inside the stator winding 8 are performed, i.e.in which the legs 10 of the “U”-shaped bars 9 are connected to oneanother or to the connection bridges 18, 28, 30, 32 and 34. FIGS. 19-25show the welding operation between the ends of two adjacent legs 10, butthe welding operation between a leg 10 and a corresponding connectionbridge 18, 28, 30, 32 or 34 is performed in a similar way.

According to FIG. 19, the welding operation is performed in a weldingstation 36 comprising an emitting device 37 which is designed to produceand emit a laser beam 38, which is suited to melt copper, i.e. the metalmaking up the stator winding 8. The welding station 36 also comprises avideo camera 39, which is designed to capture digital images of theelements that have to be welded together (the legs 10 of the “U”-shapedbars 9 in the embodiment shown in FIG. 19). According to a preferredembodiment, the welding station 36 comprises an illuminating device 40,which is suited to illuminate the elements that have to be weldedtogether (the legs 10 of the “U”-shaped bars 9 in the embodiment shownin FIG. 19); preferably, the illuminating device 40 has an annularshape, is arranged around the lens of the video camera 39, and is suitedto emit a red light (which enables an optimization of the vision of theelements to be welded during the welding operations) by means of a LEDmatrix.

According to a preferred embodiment shown in FIG. 19, a compressiondevice 41 is provided, which is mechanically coupled to the legs 10 thathave to be welded to one another, so as to push the legs 10 one towardsthe other and therefore arrange the legs 10 themselves in close contactwith one another in order to help establish an optimal electricalconnection. The compression device 41 comprises two jaws 42 which arearranged on opposite sides of the group of legs 10 and are mechanicallyconnected to one another so as to tighten the legs 10 between them.

FIGS. 20-25 show how the ends of two adjacent legs 10 are welded in thewelding station 36; the two metal elements that have to be welded to oneanother, i.e. the two legs 10, are arranged one beside the other andhave respective upper surfaces 43, which are arranged coplanar to oneanother and have at least one common edge 44.

At first, and according to FIG. 20, the two adjacent legs 10 that haveto be welded to one another are pushed one towards the other by the jaws42 of the compression device 41. FIG. 20 schematically shows only thetwo legs 10 that are to be welded to one another and the jaws 42 aredirectly pressed against both legs 10, while the jaws 42 normallycompress a group of four legs 10, as shown in FIG. 19. As shown in FIG.21, through the action of the jaws 42 of the compression device 41, thetwo legs 10 come into close contact, thus eliminating undesired gaps, ifthere are any between the two legs 10 in correspondence to the commonedge 44.

Subsequently, and as shown in FIG. 22, the video camera 39 captures adigital image of the two upper surfaces 43 of both legs 10. Then acontrol unit analyzes the digital image, in order to both determine theposition of the external edges of the two upper surfaces 43 (highlightedwith a dotted line in FIG. 22), and determine the position of thecorners of the external edges of the two upper surfaces 43. By using theposition of the corners of the external edges of the two upper surfaces43, the control unit determines the position of the center (namely ofthe barycenter) of the two upper surfaces 43 (indicated by a dotted linecross in FIG. 22). The external edges of the upper surfaces 43 and thecenter of the two upper surfaces 43 constitute geometric references thatthe control unit uses to control the position of the laser beam 38 (thatis to control where the laser beam 38 hits the upper surfaces 43).

By using the geometric references obtained from the analysis of thedigital image of the two upper surfaces 43 of both legs 10 and as shownin FIG. 23, the control unit controls the emitting device 37 in order toaim, at the upper surfaces 43 of the two legs 10, a laser beam 38, whichis suited to melt copper (i.e. the metal making up the legs 10) and isfed along a welding line 45 which is perpendicular to the common edge 44and extends through the common edge 44 itself. According to a preferredembodiment, the position of the welding line 45 is determined based onthe position of the center of the two upper surfaces 43, while theorientation of the welding line 45 is determined based on the externaledges of the two upper surfaces 43 (in particular, laser beam 38production is controlled in such a way that the laser beams 38 areparallel to the external edges of the two upper surfaces 43 which areperpendicular to the common edge 44). The laser beam 38 drawing thewelding line 45 on the upper surfaces 43 of the two legs 10 causes partof the metal making up the legs 10 to melt, thus determining theformation of a molten metal bath 46 which places itself on both sides ofthe welding line 45.

By using the geometric references obtained from the analysis of thedigital image of the two upper surfaces 43 of both legs 10 and as shownin FIG. 24, the control unit controls the emitting device 37 in order toaim, at the upper surfaces 43 of the two legs 10, a laser beam 38, whichis suited to melt copper (i.e. the metal making up the legs 10) and isfed along a welding line 47 which is perpendicular to the common edge44, extends through the common edge 44, is parallel to and spaced apartfrom the line 45 and has a length that is smaller than the one of theline 45. According to a preferred embodiment, the position of thewelding line 47 is determined based on the position of the center of thetwo upper surfaces 43, while the orientation of the welding line 47 isdetermined based on the external edges of the two upper surfaces 43 (inparticular, laser beam 38 production is controlled in such a way thatthe laser beams 38 are parallel to the external edges of the two uppersurfaces 43 which are perpendicular to the common edge 44). The laserbeam 38 drawing the welding line 47 on the upper surfaces 43 of the twolegs 10 causes part of the metal making up the legs 10 to melt, thusdetermining the formation of a molten metal bath 48 which places itselfon both sides of the welding line 47 and merges with the previouslygenerated molten metal bath 46 (thus forming a single molten metalbath).

The fact that the welding line 47 is shorter than the welding line 45allows the weld to be well balanced, because, even if there is a shorterwelding line 47, the molten metal bath 48 has substantially the samevolume as the molten metal bath 46; as a matter of fact, the weldingline 47 is performed when the upper surfaces 43 of the two legs 10 havealready been heated by the previous welding line 45, and therefore, eventhough it delivers an overall smaller quantity of heat, it causes asubstantially equal quantity of metal to melt. Thanks to the rightbalancing of the two molten metal baths 46 and 48 (obtained bydifferentiating the lengths of the two welding lines 45 and 47), it ispossible to perform a large weld (therefore a mechanically robust andelectrically low resistance weld) without the risk of molten metal burrsforming outside the upper surfaces 43 of the two legs 10; said moltenmetal burrs can be very dangerous because they can cause undesired andpotentially destructive short circuits inside the stator winding 8.

According to a further embodiment of the present invention, which is notshown, in some cases it is also possible to use a further (third)welding line, which is perpendicular to the common edge 44, extendsthrough the common edge 44 itself, is parallel to and spaced apart fromthe welding line 47 and has a length that is smaller than the one of thewelding line 47.

According to a preferred embodiment, the welding lines 45 and 47 are notcentred relative to one another. In particular, the welding line 45 issymmetrically divided by the common edge 44, while the welding line 47is asymmetrically divided by the common edge 44; in this way the risk ofmolten metal burrs forming outside the upper surfaces 43 of the two legs10 can be further reduced. In the embodiment shown in the accompanyingdrawings, each welding line 45 and 47 extends from a starting pointarranged on a leg 10 to an arrival point arranged on the other leg 10,the starting points of the two welding lines 45 and 47 are arranged atthe same distance from the common edge 44, and the arrival points of thetwo welding lines 45 and 47 are arranged at different distances from thecommon edge 44.

Owing to the above, it is clear that the laser beams 38 producing thetwo welding lines 45 and 47 are generated in (quick) succession by asingle emitting device 37.

As shown in FIG. 25, once the molten metal baths 46 and 48 havesufficiently cooled down, the jaws 42 of the compression device 41 areuncoupled from the legs 10, thus putting an end to the weldingoperations. While the welding is being performed, the jaws 42 of thecompression device 41 push the two legs 10—and keep them pushed—towardsone another along a pushing direction that is perpendicular to thecommon edge 44. Said push is generated before starting to emit the laserbeams 38 and is normally kept up until the molten metal baths 46 and 48have at least partially cooled down.

According to a preferred embodiment, the two jaws 42 of the compressiondevice 41 have an annular shape, are mounted on the stator winding 8before beginning to weld and are removed from the stator winding 8 atthe end of the welding process. An internal jaw 42 is arranged on theinside of the stator winding 8 in direct contact with the most internallegs 10, and an external jaw 42 is arranged on the outside of the statorwinding 8 in direct contact with the most external legs 10, so as topush the most external legs 10 towards the most internal legs 10 andvice versa.

A description has been provided of the ways in which laser welding isperformed, with reference to FIGS. 19-25, to weld the ends of twoadjacent legs 10, but similar ways are also used to weld a leg 10 to acorresponding connection bridge 18, 28, 30, 32 or 34; with reference tothe above-described ways in which laser welding is performed, the use ofa compression device is not provided, because the seats 19 of theconnection bridges 18, 28, 30, 32 or 34 are plastically deformedbeforehand, so as for them to be tight around the ends of thecorresponding legs 10.

The above-described ways in which laser welding is performed havenumerous advantages, since they allow the welding process to beeffective (i.e. a process resulting in a mechanically robust,electrically low resistance and long lasting weld), efficient (i.e. withvery limited processing times) and simple (i.e. which can easily beautomated and performed with precision without adopting any specialmeasures). The above-described ways in which laser welding is performeddo not require a prior peeling (i.e. removing of the external insulatinglayer) of the final ends of the legs 10 in correspondence to the uppersurfaces 43; in other words, in order to perform laser welding asdescribed above, it is not necessary to peel (i.e. remove the externalinsulating layer of) the final ends of the legs 10 in correspondence tothe upper surfaces 43.

The electric machine 1 shown in the accompanying drawings is asynchronous electric machine with permanent magnet rotor, but,obviously, the electric machine 1 may be of any other type (for exampleasynchronous and therefore without permanent magnets in the rotor).

The invention claimed is:
 1. A method to construct an electric machine(1), which comprises: a stator (5) comprising a magnetic core (6)longitudinally crossed by a plurality of stator slots (7); and a statorwinding (8) having a number of phases and comprising a series of rigidbars (9), which are “U”-shaped and are inserted through the stator slots(7) defining an entry side (12), in which the cusps (11) of the“U”-shaped bars (9) are placed, and an exit side (13), in which the legs(10) of the “U”-shaped bars (9) are placed; the construction methodcomprises the steps of: twisting the legs (10) of the “U”-shaped bars(9) coming out from the exit side (13) of the magnetic core (6) throughrelative double folds, each of which comprises an internal fold (15) inone direction and an external fold (16) in the opposite direction;electrically connecting to one another the ends of the legs (10) of the“U”-shaped bars (9) so as to form the electrical paths of the statorwinding (8); electrically connecting the ends of at least twonon-adjacent legs (10) by means of a connection bridge (18; 28; 30; 32;34), which is made of a flat plate arranged perpendicular to the centralaxis (3) of rotation and has, for each leg (10), a corresponding“U”-shaped seat (19), which is suited to receive the end of the leg (10)itself; initially manufacturing the seats (19) of the connection bridge(18; 28; 30; 32; 34) with a size that is larger, beyond the toleranceneeded to simply insert the ends of the legs (10), than the ends of thecorresponding legs (10); inserting into the seats (19) of the connectionbridge (18; 28; 30; 32; 34) the ends of the corresponding legs (10), sothat the end of each leg (10) is “loose” inside the corresponding seat(19); and locally and plastically deforming the connection bridge (18;28; 30; 32; 34) so as to tighten the seats (19) of the connection bridge(18; 28; 30; 32; 34) around the ends of the corresponding legs (10). 2.A construction method according to claim 1, wherein: each seat (19) isdelimited on three sides by two lateral walls and by a rear wall whichis interposed between the two lateral walls; each seat (19) is open onthe fourth side opposite the rear wall.
 3. A construction methodaccording to claim 2, wherein: at first the two lateral walls of eachseat (19) are not parallel to one another and each forms a correspondingobtuse angle with the rear wall; and once the end of the leg (10) hasbeen inserted into the seat (19), the seat (19) is caused to shrink,thus bringing the two lateral walls closer to each other and thereforethe two lateral walls become parallel to each other and each forms acorresponding right angle with the rear wall.
 4. A construction methodaccording to claim 1, and comprising the further step of welding theconnection bridge (18; 28; 30; 32; 34) to the ends of the correspondinglegs (10) after the step of locally and plastically deforming theconnection bridge (18; 28; 30; 32; 34).
 5. A construction methodaccording to claim 1, wherein: the connection bridge (18; 28; 30; 32;34) is made up of a central body with a substantially constantcross-section and of a series of appendages, which project From thecentral body and are perpendicular to the central body itself; and eachseat (19) is delimited, on the rear side, by the central body and islaterally delimited by a pair of appendages that are arranged one infront of the other.
 6. A construction method according to claim 5,wherein, in each seat (19), an appendage has a linear shape in its planview and the other appendage is “L”-shaped in its plan view, so as toembrace the end of the corresponding leg (10) on two sides.
 7. Aconstruction method according to claim 6, wherein the appendage with thelinear shape initially forms a right angle with the central body and the“L”-shaped appendage initially forms an obtuse angle with the centralbody.
 8. A construction method according to claim 5, wherein anappendage initially forms a right angle with the central body and theother appendage initially forms an obtuse angle with the central body.9. A construction method according to claim 1 and comprising the furthersteps of: arranging a set of first connection bridges (18), which arecoplanar to one another, at a first axial height; and arranging a set ofsecond connection bridges (28), which are coplanar to one another, at asecond axial height that is different from the first axial height, sothat the second connection bridges (28) are above the first connectionbridges (18).
 10. A construction method according to claim 9, wherein:each first connection bridge (18) has two seats (19) to connect the endsof two corresponding legs (10) to one another; and each secondconnection bridge (28) has three seats (19) to connect the ends of threecorresponding legs (10) to one another.